One way to get an idea for a story is to look at the classics and see if you can retell that story in a different time period, or to use those characters in a new way.
Think about it. Cinderella has been told a thousand times. And the Percy Jackson series is based on the Greek gods. Meg Cabot’s story Avalon High is a contemporized version of the legend of Arthur, Camelot, and Avalon.
Here’s a link to a site that has lots of information about Greek, Celtic, Norse, and other myths.
Perhaps reading through them will get your creative juices flowing!
Timeless Myths: http://www.timelessmyths.com/